The patellofemoral joint is a troublesome area in conventional total knee replacement design. A significant number of patients are left with discomfort from the patellofemoral region following total knee replacement and others have failure such as lateral patellar mal-tracking, wear through of polyethylene patellar components, fracture of patellar components, loosening of components and fracture of the patella bone itself, requiring revision surgery.
In order to obviate the point loading and high wear using a conventional domed polyethylene patellar button, U.S. Pat. No. 4,309,778 discloses a highly conforming patellar button giving area contact. However because this type of articular surface is highly conforming to the femoral component design that it is matched to, a fixed bearing of this design would not allow the normal patellar rotation which occurs in use. The design thus had to incorporate a metallic base plate and allow rotation of the polyethylene bearing component upon that base plate.
This prior art design was however found to be susceptible to failure, and in The Journal of Arthroplasty, Volume 20, No. 2, 2005, pages 202-208, an article by Huang et al. describes failures of this type of design, pointing out that this type of patellar replacement is highly sensitive to patellar mal-tracking. As there is a finite allowable thickness of patella replacement, when a (metallic) base plate is added to a patellar button design, of necessity the polyethylene articular portion has a very thin layer, making the design highly susceptible to point loading.
An object of the invention is to provide a knee prosthesis in which the patellofemoral joint is improved. A further object is to provide an improved femoral component for use in such a knee prosthesis, and also an improved patellar articular component for use in such a knee prosthesis. A still further object is to provide a device for intra-operative checking of patellar tracking to allow correct insertion of both the femoral component and the patellar component of the knee prosthesis.
According to a first aspect of the invention there is provided a femoral component of a knee replacement comprising lateral and medial condyles, an open intercondylar area between said condyles, a patellar flange, and a patellar groove extending from said open intercondylar area to a free end of the patellar flange, characterized in that the articular surface of the patellar flange for patella contact is for a distance extending inwardly from the free end of the patellar flange configured to resist forces, in use, from a medial to a lateral direction.
Preferably a twist or roller coaster configuration is built into the femoral component to resist forces from a medial to a lateral direction. Desirably the degree of twist decreases away from said free end, and conveniently the twist is terminated at a spacing from said free end of the patellar flange lying within the open intercondylar area. In a preferred embodiment the open intercondylar area is enclosed by an intercondylar box.
According to a second aspect of the invention there is provided a patellar component of a total knee replacement comprising an articular surface formed with an off-set domed region and providing a lateral facet shaped to allow area contact, in use, and a medial facet shaped to allow line contact, in use, with an associated femoral component of said total knee replacement.
The dome is off-set medially by, for example, 5 mm, but it is preferably intended to provide a range of different off-sets available, e.g. 3 mm, 5 mm or 7 mm off-set as required. The patellar component is desirably of one piece and conveniently formed of plastics material, for example polyethylene.
In full extension, the dome has point contact with the femoral component, but in full extension the patellofemoral compressive force is lowest. In slight flexion up to 45° the dome makes line contact in the floor of the patellar groove of the femoral component. At higher degrees of flexion the dome makes no contact in the intercondylar box area and on the lateral side load is taken through the lateral facet with area contact on the lateral femoral condyle of the femoral component. On the medial side, the medial facet has a cross-section similar to a ‘Mexican hat’ shape, giving line contact on the medial femoral condyle.
Advantageously the patellar component has a non-articular surface with fixation means for cement fixation. In one embodiment such means could be pegs and recesses. However the component could have a modified surface for cementless fixation.
Preferably the patellar component is manufactured by oscillating a cutter blade about the centre of the lateral facet area to produce the medial facet. This method of manufacture is according to a third aspect of the invention.
According to a fourth aspect of the invention, there is provided a total knee prosthesis comprising a femoral component of said first aspect of the invention and a patellar component of said second aspect of the invention interacting therewith, in use.
According to a fifth aspect of the invention there is provided a device for intra-operative checking of patellar tracking with a total knee replacement, the device comprising a first part having a polymeric patellar component with articular surfaces matching the femoral component of the knee replacement, and an extension member for registering with a second part of the device, the second part having a portion sized to the cut surface of the patellar and with means for engagement with said cut surface, and an extension member for registering with said first part of the device, relative sliding movement being possible, in use, between said first and second parts.
A scale on one of the first or second parts of the device indicates the relative degree of sliding between the two parts. During intra-operative use, the knee is flexed throughout the whole range of movement and the amount of patellar mal-tracking measured by observation of the scale.
As, in one embodiment, the metallic base plate part of the device, the second part, makes low friction sliding contact with the patellar component of the device, then the patella with its attached soft tissues is free medio-laterally to translate to its desired position. The tracking device thus allows the Surgeon to detect any discrepancy between where the patella ‘wants to be’ and where the patellar component ‘wants to be’ and address such discrepancy.